1. Field of the Invention
The present invention relates to a method of axially aligning a charged particle beam and also to a charged particle beam system.
2. Description of Related Art
In recent years, scanning electron microscopes for making observations and measurements on fine structures of living organisms, materials, semiconductors, and so on and charged particle beam systems such as metrology SEMs used for measurements on semiconductor device circuit patterns have become known.
In such a charged particle beam system, the axis of a charged particle beam is aligned with the optical axis of an optical member such as an objective lens.
For example, JP-A-6-176721 discloses an aligner for aligning the electron beam of a scanning electron microscope. The aligner comprises means for obtaining two filament images by controlling a condenser lens/scan coil under two sets of conditions, means for obtaining two-valued images by binarizing the two original images, a histogram creating means for obtaining a histrogram by histrogramming the two-valued images in the X- and Y-directions, means for finding two coordinates indicating a maximum value from the histogram, and means for controlling the excitation current through the alignment coil such that the distance between the two coordinates is brought to within a tolerable range.
JP-A-2005-174812 discloses a scanning electron microscope in which the axis of a charged particle beam is adjusted based on the result of a comparison between first and second images after detecting the first image by scanning the aperture of an objective lens with an electron beam in a first direction and detecting the second image by scanning the aperture of the objective lens with the beam in a second direction opposite to the first direction.
In the aligner of JP-A-6-176721, however, the excitation currents through the gun alignment coils need to be corrected repetitively until the distance between the coordinates that maximizes the two-valued images derived from filament images fall within a tolerable range.
Furthermore, in the scanning electron microscope of JP-A-2005-174812, a sequence of manipulative operations needs to be carried out repetitively until the excitation currents through the beam alignment coils reach their optimum value.